The present invention relates to ion sources having a single ionization chamber in which the plasma is created by excitation of a gas by means of a high frequency electric field, for forming either plasmas or ion beams.
Such ion sources prove to be in greater and greater demand, on the one hand, for the mechanical surface treatments of materials, for giving them an increased resistance to corrosion and wear and, on the other hand, for their application in the manufacturing phases of high integration integrated circuits, for example for cleaning the substrates, depositing thin layers (by epitaxy or for forming passivation layers), for the stripping of resins and etching the patterns.
The invention provides an ion source providing wide section beams, uniform over substantially the whole of their section and stable in time, from gases which are chemically reactive either with the materials of the discharge, or therebetween.
The invention aims at providing a satisfactory compromise between the different requirements for such sources, such as a long life span, low pollution, high yield or degree of ionization, simplicity of use and automation, reproducibility.
Ion sources having a single ionization chamber are known using the following techniques:
(A) Ion sources having a single ionization chamber are known in particular whose plasma is created by high frequency excitation by means of an alternating electric field applied between two electrodes disposed opposite each other and of different forms (flat, cylindrical or spherical), depending on the high frequency diode type with capacitive coupling.
This structure is widely used at the present time in plasma technology because of its simplicity and because it is suitable for the ionization of reactive gases, without any problem of durability. On the other hand it has deficiencies in so far as the following are concerned - the uniformity of the plasma; the pollution of a target, carried by the anode, following spraying of the cathode under the impact of the ions; the degree of ionization of the plasma; the maximum operating pressure; the ionization yield is very low; and control of the energy of the ions arriving on one or other of the electrodes, both participating in the creation of the plasma and consequently having no degree of freedom in so far as their biasing is concerned.
Some improvements have been obtained by applying a magnetic field (axial or transversal), but to the detriment of uniformity, and by using an independent biasing circuit with a different frequency for the second electrode.
Such high frequency diodes with capacitive coupling are described for example in the book by Brian Chapman "Glow Discharge Processes" 1980, John Wiley and Son, New York.
(B) Furthermore, ion sources having a single ionization chamber are known whose plasma is created by the application of a static DC electric field between a hot electron emitting filament and an anode forming the walls of the chamber, at the periphery of which magnetic confinement of the multimirror type is provided by means of alternate magnetic poles which give a multipole DC diode structure.
These sources give a dense, uniform and calm plasma over wide sections under good conditions of simplicity, reproducibility and yield; on the other hand, operation with reactive gases proves practically impossible, not only because of the vulnerability of the hot filament, with respect to oxygen in particular, but because of the possible consequences of deposits, in the chamber, of compound gases, (fluorinated or chlorinated), operation with compound gases of the fluorocarbon type leading for example to the extinction of the continuity discharge after only a few hours of operation, during which a permanent drift of the operating point is observed. Moreover, pollution of the targets occurs because of the evaporation, spraying and possible chemical attack of the hot filament.
An ionization source of this type is described in an article by T. D. Mantei and T. Wicker, Appl. Phys. Letters, 43 (1983) 84 and the articles by H. R. Kaufman, J. of Vac. Sc. Technol., 1978, vol. 15, p. 272 and H. R. Kaufman et al, J. of Vac. Sc. Technol., 1982. vol. 21?p. 725.
(C) Ion sources having a single ionization chamber are also known in which the multimirror magnetic confinement is associated with the ionization or excitation of the neutrals by an alternating electric field either of high frequency, or of very high frequency for associating the advantages and reducing the disadvantages which may result from the structures mentioned above under A and B.
Such sources are described for example in the following articles:
E. R. Ault and K. R. MacKenzie, Rev. Sci. Instrum, 44 (1983) 1967,
Fosnight et al, EDB 700205 TEW, Redondo Beach, Ca. (U.S.A.), 1982,
Y. Arnal et al, Appl. Phys. Lett., vol. 45 (1984 p. 132-134.
Similarly, the U.S. Pat. No. 4,481,062 describes an ion source having a multipole magnetic confinement chamber with a hot cathode, heated by DC current or AC current from the mains, and an anode, a DC current source applying a DC potential difference between cathode and anode.
In so far as the multimirror magnetic confinement is concerned, ion sources have also been proposed having two or three ionization chambers, for example in the French patent application No. 2,550,681 filed on the 12th August 1983 by the applicant and publically available since the 15th Feb. 1985, confinement of the ions and of the electrons in the third ionization chamber (when there are three chambers) being obtained by a multimirror magnetic configuration formed by rings or rows of alternate magnetic poles disposed about this chamber.
This type of source is very suitable for operation with chemically reactive gases which do not give rise to isolating deposits, for in such a case excitation by a DC electric field of the plasma of the third chamber has the same drawback as the one described in connection with the sources of type A. In addition, it should be noted that, even if such a possibility is ruled out, the three chamber source thus obtained is goverend by a large number of parameters and that, in exchange for its good performances (uniformity, stability of the plasma and ionization yield), it is more delicate to control and consequently to automate.
The German patent application No. 31 34 337 initially published (DOS) on Mar. 24, 1983 should also be mentioned which describes an ion gun using a high frequency going up to 2.times.10.sup.6 kHz between electrodes in the ionization chamber. No magnetic confinement device of the multipole type is provided in this patent application.
Finally, Guarnieri and Kaufman described in IBM Technical Disclosure Bulletin, vol. 24, no 11B, Apr. 1982, p. 5833-5835 (New York) high frequency excitation in a two electrode chamber, also without multipole magnetic confinement.